All transformers create a sound, which varies according to sizes and styles of the core or coil assembly. The transformer cores generally play a main role in the transformer hum. Electromagnetic forces in windings often called load noise, among other electrical and mechanical factors, may have a significant influence on low-level sound transformers.

Transformer noise is caused by a phenomenon that causes a magnetic sheet of steel to spread when magnetised. When the magnets are removed, they return to their original state. The phenomenon is known as magnetostriction effect. The alternating voltage and current, magnetically allow a transformer to stretch and contract periodically during the magnetisation process.

The magnetisation of each element in the sheet is different. Therefore the extension and contraction are not uniform. Several special steel sheets allow a transformer core to reduce loss and mitigate the resulting heating effect. The extensions and contractions occur throughout the surface.

The movements are not apparent to a naked eye. Yet they are enough to create a vibration and therefore make noise. The addition of voltage to a transformer results in a magnetic flux or magnetic force lines in the core. The level of flux decides the amount of magnetostriction and, therefore, the level of noise.

The magnetostriction cannot be removed completely, but the right design, assembly and installation of the transformer helps to control it, and likewise to mask the acoustic.